Iphone 7S Telstra plan wont send international SMS

My iphone7S under a Telstra plan will not send an SMS to a UK sim phone number. It can send messages to australian mobile numbers both iphones and android. The Sim can send an SMS to the UK number when it is put in an Android phone. Telstra says its an Apple issue and recommended doing a factory reset for the iphone.

The signal is weak here and I use Telstra Wifi Call

Is there anything else I can do?